Acoustophoretic printing apparatus and method

ABSTRACT

The present invention contains a printing apparatus and a method, e.g., for ejecting inks (i.e. pure liquids, mixtures, colloids, etc.) for a very broad range of physical properties (such as viscosity). Acoustic forces  3   a  may be generated by an emitter  1  and a reflector  2  to detach droplets  10  from a nozzle  6.  The ink may be advanced through the nozzle  6  by a standard back pressure system  5.  A reflectorless chamber  7  may enhance acoustic forces  8   a  and the droplets  10  may be ejected at a bottom  9  of said chamber  7,  so that droplets  10  may be deposited on any substrate  11.

The present invention relates to a printing apparatus and a printingmethod for ejection of inks from a nozzle using acoustic forces. One ofthe technical advantages is that inks of any type and temperature may beprinted, which alleviates severe limitations of the current state of theart.

To date, the largest technological limitation in planar and 3D-printingis the ability to dispense materials over a wide range of physicalproperties, as described by B. Berman, Bus Horizons 55 (2012).

Today, the technologies of choice for the printing of multiple inks arelimited to extrusion and inkjet processes (S. Upcraft, and R. Fletcher,Assembly Autom 23 (2003); A. Jaworek, and A. Krupa, Journal of AerosolScience 30 (1999)).

The extrusion process “pushes” a very viscous ink through a smallnozzle, very similarly to a toothpaste, while the inkjet process relieson a dynamic ejection principle based on strong pressure wave within thenozzle, generated usually by a piezoelectric device (B. Derby, AnnualReview of Materials Research, Vol 40 40 (2010)).

However, to-date-printing technologies are limited and requirecumbersome ink preparation to achieve a degree of functionality. Inextrusion printing, the push (pressure) required to eject relativelysmall features becomes so large that is not practically possible toeject the ink. As a result, it works either with high temperature—toreduce the viscosity of the material at ejection as described by S.Uperaft, and R. Fletcher, Assembly Autom 23 (2003)—or with specificnon-Newtonian fluids that exhibit a shear-shinning behavior (e.g.,thixotropy, the higher the stress they experience the lower theirviscosity) which is described by K. Sun et al., Adv Mater 25 (2013)).

On the other hand, inkjet technology is powerful and it is integrated intoday's society (i.e., the inkjet printer). However, this inkjetdrop-on-demand (DOD) technology is severely limited to fluids withviscosity approximately 10 times that of water (B. Derby, Annual Reviewof Materials Research, Vol 40 40 (2010)).

From a physical point of view, the printing process can be characterizedby a non-dimensional number, the Ohnesorge number Oh, and its inverseZ=Oh⁻¹,

Z=(ρσd)^(1/2)/μ

with ρ, σ and μ being the density, the surface tension and the viscosityof the ink, respectively, and d the characteristic droplet size(typically the nozzle diameter).

J. E. Fromm, Ibm J Res Dev 28 (1984) as well as N. Reis, and B. Derby,Materials Development for Direct Write Technologies 624 (2000) reportthat the inherently rapid dynamics of the inkjet printing allows only anarrow window of possible physical properties: literature reports amaximum range of 1<Z<10.

The constraints of to-date-printing technology means that, for instance,with inkjet technologies even water cannot be reliably ejected: itsviscosity is indeed too low (Z≈60 for droplets of 50 μm in radius). Inbiology, most of the practical liquids are water-based solutions withvolumes in the μl-ml range; indeed pipetting is still the gold standardtechnique for dispensing in the μl-ml range. At the lower limit of Z(Z≈0.04), glycerol, a cornerstone ingredient for food and pharmaceuticalindustries, is too viscous to be ejected.

In general, most inks of interest to realistic applications are basedupon colloids or polymers that have relatively high viscosity, anddilution is usually the only available, but not-optimal solution (B. J.de Gans, P. C. Duineveld, and U. S. Schubert, Adv Mater 16 (2004)).

As a result of such limitations of the current state of the art inprinting methods, a large effort is being expended today to engineerinks matching the limited Z-window of the printer. For instance,additional techniques and preparation steps such as photopolymerization,thermal curing, annealing and additives are investigated or used,respectively.

Summarizing, there is a need for decoupling the printing process fromthe physical properties of the ink, which would give an unprecedentedfreedom in choosing the materials that can be printed.

The here-presented printing apparatus and printing method according tothe independent claims solves the above-discussed problems. Thedependent claims describe preferred further developments.

The printing apparatus according to one aspect may comprise an emitter,which may be arranged within a first fluid. The emitter may beconfigured to oscillate so that an acoustic field in said first fluidmay be generated. Preferably, the emitter may oscillate periodically.The printing apparatus may further comprise a nozzle, which may beplaced at a predetermined position within said acoustic field. A secondfluid may be arranged within the nozzle.

Further, there may be provided a driving means, which may be connectedto the nozzle. The driving means may be configured to drive the secondfluid, a predetermined volume, out of the nozzle. In other words, adroplet of second fluid may be driven out of the opening of the nozzle.

Preferably, the driving means may be configured to apply a (back)pressure on the second fluid for pushing out (of the nozzle) apredetermined volume of the second fluid. Alternatively or in addition,the driving means may be a means for reducing the pressure of the firstfluid such that the second fluid is sucked out of the nozzle. The drivemeans may also reduce the viscosity of the second fluid, e.g. thermally,so that the pendant droplet is formed. Also it may change thewettability of the nozzle opening so that the second fluid is driven outby, e.g., the gravity.

Driving the second fluid out of the nozzle, in particular and forexample, means to form a pendant droplet at the nozzle tip.

The first and second fluid may be any fluid at any temperature (liquidand gas). Preferably, the first fluid may be air and the second fluidmay be a liquid, such as water, an ink/dispersion, a liquid metal or thelike. The ink may be, for instance, a pure liquid, a mixture, a colloid,etc. The air may be at high or low temperature. Preferably, first andsecond fluids both may have room temperature, however, any othertemperature is possible. The pressure of the first fluid can possibly beatmospheric but other pressures are feasible. Further, the first and thesecond fluid may have different temperatures, e.g. the second fluidwithin the nozzle may be heated or cooled by a heating/cooling meansattached to the nozzle or to a fluid reservoir (within the drivingmeans). Alternatively, the first and the second fluid both may beliquids. For example, printing of liquid within an immiscible liquid ispossible. In case of liquid-in-liquid printing, the acousticmedium/first fluid may be oil, for instance.

The emitter may be a plate-like member. The emitter may beaxi-symmetric, two-dimensional (extrusion in the plane) or otherwiseshaped. Preferably, the shape allows for the formation of a standingwave between the emitter and an optional reflector. More preferably, anemitter with a circular, squared, hexagonal, rectangularmember/(cross-)section or the like may be used. The emitter can have aflat, convex or concave surface.

The emitter(s) may oscillate(s) periodically with any waveform.Preferably, the oscillation waveform has a sinusoidal shape, e.g. in anultrasonic range. The emitter may emit sound waves, which may beproduced/generated by the oscillations of the emitter in the firstfluid. The sound waves may form an acoustic field, which, thus, may begenerated by the oscillating emitter.

The oscillation frequency preferably may be in the range of 1 Hz to 1GHz, and more preferably in the range of 10 Hz to 100 MHz. Thewavelength λ refers to the wavelength of the emitted/reflected acousticwave, which is the result of speed of sound c (in the first fluid)divided by the oscillating frequency of the emitter. The wavelength λ ofthe emitted/reflected acoustic wave between emitter and reflector may bepreferably in the from range 1 μm to several meters. Frequencies above16 kHz (close to the ultrasound range, which is not audible by humanhearing) and below 1 MHz offer a most preferred compromise betweenacoustophoretic printing feature size, strength of acoustic forces, andcomponent manufacturing.

The oscillation velocity amplitude of the emitter may range from 1 μm/sto 100 m/s.

The emitter(s) may be attached to or may include a piezoelectrictransducer, a magnetostrictive transducer or any other means that canprovide the needed wave excitation. For example, the emitter may be aplate-like member, which is periodically driven by the piezoelectrictransducer connected to it. The emitter may also be an integrated unit(i.e., an oscillator) which comprises the transducer, the plate-likemember and preferably further components, such as electric connectionsand the like.

The nozzle may be a means for holding the second fluid therein or it maybe connected to such a means. Preferably, the nozzle may be part(integral or not) of a capillary or any other kind of a tube, and,further, the nozzle may have a tip with a diameter being smaller thanthe remaining part of the nozzle/capillary. The nozzle tip (and therespective opening) may have a diameter ranging from 0.01 μm to severalmillimeters. More preferably, the nozzle tip (and its opening) may havea diameter in a range from 0.01 μm to 250 μm. Most preferably, a nozzletip (and its opening) with a diameter in the range of 1 μm to 250 μmoffer the best compromise between a minimum acoustophoretic printedfeature size and pressure drop within the nozzle.

The nozzle may be connected to a tubing for a fluidic connection betweena second fluid reservoir and the nozzle (tip). The tubing may have adiameter ranging from 0.1 μm to several centimeters.

The nozzle (tip) may be made of any material. Preferably, it may be atapered glass capillary, a metallic tube, a Teflon capillary or amicrofabricated tube. If a glass capillary is used, it may be useful tocarry out a hydrophobic treatment, depending on the second fluid, inparticular when an ink is printed. For water-based inks (e.g. most ofthe biological solutions and the currently emerging environment-friendly3D printing technologies), the wetting of the nozzle (tip(s)) by theink(s) is reduced by using hydrophobic treatment.

The hydrophobic treatment may also be used for reducing the capillaryforce Fc. Reducing the capillary force Fc allows for using smalleracoustic forces.

To advance the second fluid, e.g. ink, through the nozzle, a drivingmeans, such as a conventional pressure device/system may alreadysuffice. The pressure may be, preferably, a back pressure, wherein theterm “back” indicates a physical position at an opposite side of a(droplet of) the second fluid with respect to the nozzle opening. Inother words, if a volume of second fluid is enclosed within the nozzle,the pressure at the nozzle tip/opening would be smaller than the (back)pressure at the opposite side of the volume of second fluid (so that thesecond fluid is driven in the direction of the nozzle tip). For example,the driving means may include or be a syringe-pump or a pressurecontrolled (second fluid) reservoir. Further, the driving means mayinclude a (manually, electrically, thermally, and/or mechanically)squeezable member or any other means that would cause a negativepressure gradient in the direction of the nozzle opening (decreasingpressure in the direction of the nozzle opening).

Pushing a predetermined volume of second fluid out of the nozzle (or thenozzle tip/opening, respectively) shall mean that second fluid is drivenout of the opening of the nozzle tip so that a pendant drop(let) isformed. The pendant droplet may have an outward-bent meniscus. The backpressure preferably does not drive the droplet to detach from thenozzle; it merely squeezes second liquid out of the nozzle tip/openingso that the acoustic forces of the acoustic field, which surrounds thenozzle tip, may act on the pendant droplet.

The acoustic forces may (mainly) cause the detaching of thependant/pushed out droplet. This may be achieved by activating theoscillation of the emitter at the point in time at which the dropletshall be detached. Additionally or alternatively, the oscillation of theemitter may be modulated at the point in time at which the droplet shallbe detached, for example the modulation may cause the emitter to changethe oscillation, e.g. frequency, phase and/or amplitude (in case ofmultiple emitters, those parameters may be set differently for eachemitter), such that acoustic forces are generated which are large enoughfor detaching of the droplet.

Additionally or alternatively, the oscillation of the emitter can be setconstant during the single droplet ejection process. In such a way, amean-force due to the acoustic radiation pressure can be exerted on thependant drop that is growing in time (it is pushed/squeezed out of thenozzle). When the droplet is large enough so that the acoustic forcesovercome the capillary forces, detachment can be achieved. In all timemodulation of the force during the ejection process is possible asneeded.

Summarizing, a controlled ejection of predetermined volumes atcontrolled frequency of droplet ejection is enabled by described use ofacoustophoretic forces. The proposed acoustophoretic printing is basedon material-independent acoustic/acoustophoretic forces that allow Znumbers in a very broad range from 10⁻³ to 10³. The acoustic forces areharmless to cells so that a wide range of biological applications ispossible. It is further noted, that the printing apparatus/method thatis presented here is useful in a wide variety of different printingapplications such as those pertaining to complex fluids for flexibleelectronics, novel micro and nanofluidics technologies, printed energyharvesting and sensing technologies, and also for a wide range ofbiological applications ranging from wearable and implantablediagnostics and medical devices to bio-synthetic organ printingapplications.

The ejected volume can be varied by more than three orders of magnitudeby controlling the acoustic forces. In other words, the printing processof the described printing apparatus is decoupled from the physicalproperties of the second fluid.

The use of an optional reflector, which may be arranged between theemitter and a printing substrate, is a relatively straightforward wayfor forming a standing wave between the emitter and the reflector.Forming a standing wave has the technical advantage that the acousticforces are higher than in an acoustic field that does not form astanding wave. However, the presently described printing, generally, isalso feasible in an acoustic field that does not form a standing wave.

The printing substrate may be any object or member on which the printingmaterial (second fluid) shall be deposited/printed. The geometry of thesubstrate is not restricted to flat planar surfaces, but may bearbitrarily shaped.

Placing the reflector between the printing substrate and the emittershall not exclude that further components or objects may be placedin-between. The emitter, the reflector and/or the substrate do not needto be connected to each other. However, a frame or the like may connect,in particular, the reflector and the substrate with each other. Thecomponents of the printing apparatus may also be placed within a casing.

Preferably, a surface of the reflector may be positioned at a (defined)distance H from (a surface of) the emitter. More preferably, thedistance H between the surfaces of the emitter and the reflector may bea multiple of λ/2±0.2λ, e.g. 0.5λ±0.2λ, 1.0λ±0.2λ, 1.5λ±0.2λ, etc.Setting the distance H accordingly enables imposing an acoustic standingwave between the emitter and the reflector. Typically, a distance H≈λ/2offers the strongest acoustic standing wave in regard of acousticforces.

The surface of the reflector, which may be placed at the half of thewavelength λ, may preferably be an upper surface of the reflector. Thissurface of the reflector may oppose a (lower) surface of the emitterfrom which sound (acoustic) waves are emitted. In case that the emitteris placed below the reflector, the surfaces would be swapped.

The geometry of the reflector may be axi-symmetric, two-dimensional(extrusion in the plane) or it may have any other geometry that allowsfor the formation of a standing (first) wave between the emitter and asurface of the reflector. Preferably, an emitter with circular, squared,hexagonal or rectangular (cross-) section/surface may be used and areflector with a characteristic dimension of its (cross-) section (e.g.the upper surface) of the same order of magnitude.

Moreover, different combinations and configurations of the emitter andreflector setup are possible: the emitter and the reflector may beplaced such as described above. The reflector may be entirely omitted. Aplurality of emitters may be arranged next to each other or along ashared longitudinal axis. The emitter and reflector may be placed eitherup or down.

The reflector may have a reflectorless chamber (or conduit) which maypreferably be positioned along a printing axis between the emitter ornozzle tip and the printing substrate. More preferably, thereflectorless chamber may include or be a through-hole through thereflector; accordingly, the reflector may be hollow.

The term “reflectorless” shall mean, in particular, that there is noreflector arranged along the printing axis or the entire longitudinalaxis of the reflectorless chamber, wherein, preferably, the printingaxis and said longitudinal axis may be identical or parallel. Theprinting axis may be (or at least stretch along) an imaginary/virtual(preferably straight) connecting line between the opening of the nozzleand a point on the substrate on which the second fluid/droplet shall beprinted/deposited. The printing axis/vector does not necessarily have topoint in the direction of the gravitational force. The nozzle axis doesnot necessarily have to be parallel to the printing axis, it may bearranged with an angle with respect to the printing axis. If there is anangle between the printing axis and the nozzle axis, preferably, it maybe between >0° and <=90°.

A characteristic dimension of the reflectorless chamber may be thediameter in case, e.g., the conduit/reflectorless chamber would have acircular geometry or a side length, e.g., in case the reflectorlesschamber would have a square geometry, etc. The reflectorless chamber mayalso have a varying cross section (along its entire length). Thereflectorless chamber may also be square, hexagonal, elliptical or thelike. It can be smaller at the entrance and bigger at the exit and viceversa. The most general description of the reflectorless chamber may bethat it should be a confined space with an opening at its top and at itsbottom.

The characteristic dimension may be smaller or equal to the wavelength λof the first standing wave between the emitter and the (upper orrelevant) surface of the reflector. More preferably, the characteristicdimension of the reflectorless chamber may be in the range of 0.01λ toλ.

The configuration in which the characteristic dimension equals orexceeds the wavelength λ may also represent a manifestation of theherein-described printing apparatus, which does not have the optionalreflector. In this case, the acoustic field/sound waves do notnecessarily form a standing wave so that the acoustic forces may beincreased by increasing the amplitude of the acoustic waves or the like.Further, as described above, the acoustic forces may also be increased,in particular if no reflector is used/no standing wave is formed, byfocusing sound waves of a plurality of emitters on a specific point inspace (as described below). Alternatively, the printing substrate mayalso act/be used as reflector. The emitter and the substrate would bearranged such as to have the defined distance H as described above.Further, however, it may also be an alternative to have a reflector witha reflectorless chamber having a characteristic dimension that equalsthe wavelength λ.

The reflectorless chamber preferably has a constant characteristicdimension along the ejection line/printing axis—preferably a cylindricalsection with a constant radius. However, the reflectorless chamber mayalso have a varying cross section along the printing axis. For instance,it may also be conical, increasing the characteristic value whileexiting the chamber. In general, a larger characteristic value at theexit may allow for a more stable droplet ejection. The reflectorlesschamber may enhance the acoustic forces and eject droplets at the bottomof this reflectorless chamber, so that the droplets may be deposited onany object or substrate. A second standing wave may be present withinthe reflectorless chamber which is up to 1-100 times enhanced, in regardof the acoustic forces, than the first standing wave between the emitterand the reflector (the relevant surface thereof).

The height Hh of the reflectorless chamber or the entire reflector maybe in the range of 0.01λ to 100 λ. More preferably, the Hh-range for agood ejection is from 0.1 to 10 λ.

The nozzle tip may be positioned in any place/at any position within thereflectorless chamber or above or below. Typically, if Hh is the heightof the reflectorless chamber, the preferred region/position to place thenozzle tip for a reliable ejection is where a net force pulling thependant droplet is present. A preferable range for positioning thenozzle tip may be from −0.5 Hh (negative means above the chamber) to 1.5Hh (bigger than 1, below the exit of the chamber). In other words, “−0.5Hh” would mean that the nozzle tip is arranged between the emitter andthe upper surface of the reflectorless chamber/reflector at a positionwhich has a distance of a half of the length of the reflectorlesschamber from the upper surface of the chamber/reflector. “1.5 Hh” wouldmean, accordingly, that the nozzle tip is placed at a position, whichhas a distance of a half of the length of the reflectorless chamber fromthe lower surface/exit of the chamber/reflector. More preferably, thenozzle tip is arranged within the reflectorless chamber. Even morepreferably, the nozzle tip is placed within the reflectorless chamber ata position which is in the range from the exit of the reflectorlesschamber to Hh/3. In this range, in particular, a large net force pullingon the pendant droplet is present.

The printing apparatus may comprise a plurality of emitters. Theemitters may be arranged such that emitted sound waves are focused on apredetermined point in space within the (first) acoustic field. In otherwords, the acoustic field of the different emitters may be configuredsuch that the acoustic forces of the different emitters add up at thepredetermined point in space. This may be achieved by experimentallyadjusting the orientation of the different emitters. The acoustic forcesof the different acoustic fields of the emitters may also be calculatedor simulated for finding the desired arrangement. The multiple emittersmay also be used to form a standing wave in a predetermined point, whichmay be achieved in a manner similarly to focusing, but with differentemitted sound/acoustic wave characteristics of the different acousticwaves. The predetermined point in space is preferably the predeterminedposition of the nozzle tip. The predetermined position may also be inthe vicinity thereof. Here, “vicinity” may mean that the nozzle tip isplaced in an area at which the acoustic forces of the different emittersadd up. The area/vicinity may mean that the nozzle tip is located withina range of few micro- to centimeters from the predetermined point inspace.

The advantage of this aspect of the printing apparatus is, especially,that the acoustic forces of the plurality of emitters add up such thatthe acoustic forces can reach a value being sufficient for theacoustophoretic printing—even without using a reflector or a standingwave. This reduces the complexity of the printing apparatus and providesadditional degrees of freedom in regard of the configuration/design ofthe apparatus. However, it is noted, that the emitters may also bearranged such that a standing form wave is formed without a reflector,as it will be described further below.

The acoustic field may have a force gradient of the acoustic force(s)that may point into the direction of printing, and the predeterminedposition of the nozzle tip may be at a point or in the vicinity of a net(acoustic) force which pulls/draws the pendant droplet away/detaches itfrom the nozzle tip and accelerates the detached droplet. The net forceis generated by a force gradient(s) of the acoustic potential of theacoustic field. The force gradient may be adjusted by the presence ofthe droplet, the nozzle tip, the reflector, the substrate, the emitterand their shapes/geometries. The necessary gradient of the acousticforce and the necessary net force, respectively, may be calculatedand/or simulated.

For example, a gradient of acoustic forces within an added up acousticfield, preferably, may already be generated by the distances between theemitters and the point in space, at which the nozzle tip is placed. Thepresence of the nozzle tip and of the droplet may further generate oralter the force gradients. The force gradient(s), which may preferablypoint from the nozzle tip to the printing direction, i.e. acousticforces being larger at the nozzle tip than downstream the printingdirection, may be obtained by simulation, calculation and/or experiment.For instance, the force gradient can be obtained by a non-symmetricstanding wave generated by two emitters, or by an emitter and areflector shaped and oriented in a specific manner. This also holds forthe general and other configurations/examples of the printing apparatus,e.g. the configurations as set forth above.

The emitter(s) may have an integrated reflector, which shall mean thatthe reflector and the emitter are integrated into a single component.The integrated reflector may, for example, be a specifically shaped sidewall of the emitting surface (emitter surface) or the like, which mayfocus/reflect the sound waves. More preferably, said reflector may beformed such that the sound waves of the emitter are focused on apredetermined point in space, at which the predetermined position of thenozzle tip may be located (or in the vicinity thereof). The shape of thereflector may be individually adapted to the emitter of a printingapparatus, and may be simulated or calculated, for instance. E.g., theemitter can have a flat, convex or concave surface. It can also beshaped in a concave way so that the emitting surface at a point can alsoact as a (self-/integrated) reflector for another correspondent point ofthe same emitting surface.

The technical advantage of using a reflectorless chamber, in which a(second) standing wave may be formed, is that the acoustic force(s) ofthe acoustic field may be increased up to 1 to 100 times compared to the(first) standing wave in the (first) acoustic field between the emitterand the reflector. The increase is i.a. effected by the smallercharacteristic size of the reflectorless chamber compared to thespace/chamber between the emitter and reflector. The increase of theacoustic forces i.a. allows for printing smaller features sizes.

Further, a plurality of nozzles and/or reflectorless chambers may bearrayed, or arranged in an array, for forming a multi-nozzle print head.The multi-nozzle print head i.a. allows printing of a plurality ofdroplets at the same time and at the same or different points on thesubstrate.

The nozzle may have a heating means and/or a cooling means forheating/cooling the second fluid to a predetermined temperature. Themeans may also be attached to the nozzle. In this way, the temperatureof the second fluid, for example, may be increased so that the viscositymay be reduced, which may allow for reducing the acoustic forces or forprinting smaller feature sizes/droplets. The heating means may be aheating wire being wound around the nozzle. Other heating sources may beused, too. The cooling means may be a Peltier element, by thermalconduction with another solid, convection with a fluid or the like.

A print head may comprise a plurality of reflectorless chambers so thata plurality of integrated ejectors may be formed, which advantageouslyallow for serial and/or parallel printing (time- and/or spacewise). Theterm “print head” may indicate a member in which the exit(s) of areflectorless chamber(s) may be formed. For example, the print head maybe integral with the reflector or the emitter. Further, the print headmay also be a separate/additional component.

The nozzle and the drive means may be connected via a tubing. The tubingmay be introduced into the printing apparatus at a side surface thereof,preferably below the emitter. Alternatively the tubing may be introducedthrough a hole in the emitter. The tubing may include, in a cost optimalconfiguration, flexible plastic tubes so that an economic and flexibleprinting apparatus can be built.

The predetermined volume of second fluid, which may be pushed out of thenozzle opening by the back pressure, may range from nl to μl. This broadrange allows printing extremely small as well as relatively largefeature sizes. The flexibility in regard of the printable/ejectablevolume is a further technical advantage compared to the to-date printingtechnologies, which are more limited in this regard.

An (electric) control means, such as a computer or the like, may beconfigured to control at least the oscillation of the emitter, thepressure application of the drive means and/or a driving of the printingapparatus to a printing position in relation to a printing substrate.E.g., the printing apparatus may include or may be connected to amovable frame, which may be (automatically) driven to a (plurality of)printing position(s). Further, the application of the back pressure maybe automated, as well as an (de-)activating of the emitter oscillations.Hence, computer-controlled, automated printing is enabled.

A further aspect may comprise a method for ejecting a fluid from anozzle. Preferably, the method makes use of at least one example of theabove-described printing apparatuses according to the invention.Preferably, the method allows ejecting a second fluid from a nozzle,which may be arranged within a first fluid. The method may have a stepof forming a droplet at a nozzle tip. In other words, a predeterminedvolume of second fluid may be driven out of the nozzle tip. The nozzletip may be arranged within an acoustic field generated by an(periodically) oscillating emitter. The second fluid may be driven byapplying a (back) pressure on the second fluid.

Further, the method may include the step of generating an acoustic fieldwith an acoustic force gradient, which may point from the nozzle tip toa printing substrate by activating a periodically oscillating emitterand/or by modulating the oscillation of the emitter.

Summarizing, the proposed acoustophoretic printing is based onmaterial-independent acoustic forces. Such forces allow a possible Znumber spanning seven orders of magnitude, from 10⁻³ to 10³. Theacoustic forces have been shown to be harmless to cells (D. Foresti etal., P Natl Acad Sci USA 110 (2013)). Therefore, they can be used for awide range of biological applications. In addition, the ejected volumecan be varied by more than three orders of magnitude simply bycontrolling the acoustic forces.

The technical advantages of the presented acoustophoretic printing allowa use in a plurality of fields of application: In medicine, for example,3D-printing of human organs research may strongly benefit byacoustophoretic printing (K. Pataky et al., Adv Mater 24 (2012)). Inbiology, solutions of living cells in the nl-μl range may be deployedacoustophoretically, minimizing contamination (M. T. Guo et al., Lab ona Chip 12 (2012); K. Choi et al., Annual Review of Analytical Chemistry5 (2012); S. Ekins, J. Olechno, and A. J. Williams, Plos One 8 (2013)).In electronics, new low-viscosity silver reactive inks, with aconductivity equivalent to bulk silver, may be DOD printed by thisacoustophoretic technique (S. B. Walker, and J. A. Lewis, J Am Chem Soc134 (2012)). In material manufacturing, a new method may complement themicrofluidics technologies for microparticle production (H. C. Shum etal., Macromol Rapid Comm 31 (2010)).

In the following, examples are set forth with reference to the attachedschematic drawings:

FIG. 1 An acoustophoretic ejection system/acoustophoretic printingapparatus. A reflectorless chamber is halved only for illustrationpurposes. The close ups show ejected glycerol and water droplets (scalebar=500 μm).

FIG. 2a ) Acoustic levitation in a conventional (state-of-the art)standing wave levitator and acoustic force distribution. In this case,one acoustic node is present (H≈λ/2);

b) When a pendant drop is placed within the conventional standing wavelevitator, it experiences both body forces (gravitational, F_(g)) andsurface forces (acoustics, F_(a), and capillary, F_(c)).

FIG. 3 Acoustophoretic printing size control capabilities. More thanthree orders of magnitude of volume for DOD ejection can be controlledwith the here-described acoustophoretic printing apparatus/method.

FIG. 4 Two preferred alternative configurations of an acoustophoreticprinting apparatus:

Configuration A) A nozzle is introduced from a side of a chamber (with amain/first standing wave) and reaches a reflectorless chamber where asecondary acoustic standing wave is present.

Configuration B) A nozzle enters from a hole on a top of theemitter/reflector and ends in a reflectorless chamber.

FIG. 5 Multiple nozzle configuration of an acoustophoretic printingapparatus. By replicating a configuration as depicted in FIGS. 4a, 4b ,multiple droplets (also of different inks) can be simultaneouslyejected. The ejection angle allows for a fine printing resolution at thesubstrate/target surface.

FIG. 6 Two emitters are used to achieve a standing wave without areflector.

FIG. 7 Multiple emitters can be used to enhance the acoustic radiationpressure at a single focal point or to create a vortex beam.

FIG. 8 An emitter can be designed so that it can simultaneously act as areflector. Both sides oscillate from opposite direction, creating astanding wave.

FIG. 9 An acoustophoretic printing apparatus used as a sample dispenserfor biological solutions in the nl-μl range volume for standard 96, 384and 1536 well plates.

FIG. 10 An exemplary numerical simulation of the acoustic pressure (inPascal) within a preferred configuration of an acoustophoretic printingapparatus. In this particular case, the maximum acoustic pressure in theprimary chamber is ≈3300 Pa, while in a reflectorless chamber thepressure is ≈15000 Pa. Since the acoustic forces are proportional to thesquare of the acoustic pressure, an enhancement of ≈25 times isexpected.

References in the specification to a given example indicate that theexample described may include a particular feature, structure, orcharacteristic, but every example may not necessarily include theparticular feature, structure, or characteristic. It should be notedthat the description and drawings/figures merely illustrate theprinciples of the proposed apparatus and method. It will thus beappreciated that those skilled in the art will be able to devise variouscombinations and the like that are not explicitly described or shownherein. Furthermore, all examples recited herein are principallyintended expressly to be only for pedagogical purposes to aid the readerin understanding the principles of the proposed method and apparatus andthe concepts contributed by the inventors to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions. Moreover, all statements herein recitingprinciples, aspects, and embodiments, as well as specific examplesthereof, are intended to encompass equivalents thereof.

FIGS. 2a and 2b show conventional configurations of a standing wavelevitator, which are used for trapping particles or fluids at a locationof an acoustic node between an emitter 1 and a reflector 2. The emitter1 generates an acoustic field 3 of sound waves, which apply an acousticforce 3 a (radiation pressure) on objects, which are positioned withinthe acoustic field 3. In the typical configuration of a standing wavelevitator, as depicted in FIGS. 2a and 2 b, a (detached) droplet 10 istrapped at the location of the acoustic node (see FIG. 2a ). An ejectionof the trapped droplet 10 with the conventional standing wave levitatoris very difficult.

The here-described printing apparatus and the printing method make useof the same physical principle as the conventional standing wavelevitator, however, different to the standing wave levitator, allowejecting droplets, in particular, from a nozzle.

The physical principle of levitating objects by acoustic forces will bebriefly described: sound waves apply forces on objects. The acousticforces 3 a, 8 a, which are used in connection with the here describedprinting apparatus and printing method, in particular, arise fromradiation pressure. Radiation pressure is a nonlinear effect of theacoustic field 3 (E. H. Brandt, Nature 413 (2001)). Based on wavescattering, acoustic forces are practically material independent, inparticular when handling samples in air (D. Foresti, and D. Poulikakos,Physical Review Letters (2014)). Albeit radiation pressure is usuallyrelatively weak, it can levitate, e.g. in open air, objects as heavy assteel marbles. The levitation can be achieved, e.g., when properlyfocusing the radiation pressure (N. Bjelobrk et al., Appl Phys Lett 97(2010); D. Foresti et al., Sci Rep-UK 3 (2013)). To describe modulationof such acoustic forces 3 a, 8 a in time and space that can facilitatetransport of matter, the term acoustophoresis is used (D. Foresti etal., P Natl Acad Sci USA 110 (2013)).

The enhancement of such forces 3 a, 8 a is typically, but notnecessarily, achieved by generating an acoustic standing wave,established between an emitter 1 and a reflector 2 (see FIG. 2 a; and D.Foresti, M. Nabavi, and D. Poulikakos, Journal of Fluid Mechanics 709(2012)). The resonant condition suggests the distance H between anoscillating source and a reflective surface to be about a multiple ofhalf of the wavelength λ. When H≈λ/2, a pressure node is generated inthe middle of the levitator. Below the node the acoustic forces 3 a, 8 aoppose the gravitational force, and vice versa above the node (FIG. 2a). Acoustic levitation is indeed intrinsically stable.

When a drop 10 of radius R_(s) is introduced in the system, as it isexemplarily shown in FIG. 2 b, wherein the drop 10 is pendant from anozzle 6 of diameter d, the drop 10 experiences a force that is the sumof the capillary force F_(c)=πσd, the gravitational force F_(g) and theacoustic force F_(a) (FIG. 2b ).

If F_(c)>F_(g)+F_(a), the droplet 10 stays attached to the nozzle 6,otherwise the droplet 10 will detach. The acoustic force F_(a) scaleswith the volume of the drop/droplet 10 (being attached to the nozzle 6),hence F_(a) ∝ R_(s) ³. Since the same scaling applies for the gravityforce, an acoustic acceleration g_(a) can be introduced, which issimilar to the gravitational acceleration g, and the followingrelationship is obtained:

$F_{c} = {{{\pi\sigma}\; d} = {{F_{g} + F_{a}} = {{\frac{4}{3}\pi \; R_{s}^{3}{{\rho \left( {g + g_{a}} \right)}R_{s}}} = \sqrt[3]{\frac{3{d\sigma}}{4{\rho \left( {g + g_{a}} \right)}}}}}}$

The above relationship tells that the droplet size at detachment iscontrollable by controlling the acoustic acceleration g_(a), i.e., theacoustic forces F_(a).

Hence, differently from conventional inkjet printing, theacoustphoretically detached droplet 10 is pulled out of the nozzle 6by/from the external acoustic field 3 (specifically, the acoustic forces3 a, 8 a resulting therefrom) instead of being pushed by an internalpressure wave. The external acoustic field 3 is tunable. The fluiddynamics within the nozzle 6 do not limit the printing. The aboverelationship shows the independence of the printing method from theviscosity of the fluid (for Newtonian fluids).

A preferred example of the here-introduced acoustophoretic printingapparatus may be called a reflectorless standing wave levitator. Aschematic representation of is shown, e.g., in FIG. 1.

The acoustophoretic printing apparatus of FIG. 1 has an emitter 1 which(periodically) oscillates so that sound waves are emitted that form anacoustic field 3 within a first fluid (not explicitly shown). The firstfluid is preferably air. In a possibly simplest configuration, theemitter 1 is a planar or curved member with a circular, elliptic,quadratic, rectangular or the like cross-section, which oscillates. Theemitter 1 may be connected to an oscillator (not shown) whichexcites/moves the emitter 1.

Further, the acoustophoretic printing apparatus according to the exampleof FIG. 1 has a nozzle 6 being placed inside a reflectorless chamber orconduit 7. The reflectorless chamber 7 is formed as a through-hole,which extends from an upper surface 2 a of a reflector 2 to a lowersurface 2 b of the reflector 2. FIG. 1 shows a preferred example of anannular-shaped reflector 2. The axis of symmetry of the reflector 2 isarranged on a printing axis A (see FIG. 4), along which droplets 10 ofsecond fluid are ejected. The surfaces 2 a, 2 b are shown as planarsurfaces. Said chamber 7 “traps” a (second or secondary) acousticstanding wave 8.

The reflector 2 of said preferred configuration of an acoustophoreticprinting apparatus may have one or more of the following threecharacteristics: (1) There is no reflector on top or bottom of thenozzle axis; hence, the name reflectorless. In other words, thereflectorless chamber 7 connects two openings being arranged within theupper and lower surface 2 a, 2 b of the reflector 2. (2) The acousticforce 3 a, 8 a (inside the chamber 7) is enhanced up to 1-100, e.g. 30times, compared to the above-described typical conventional levitatorconfiguration as shown in FIG. 2 a. (3) A force gradient of the acousticforces 3 a, 8 a, in particular with a top force stronger than a bottomforce (directions according to FIG. 1), is present. A net force is,i.a., present because of the optimal design of the reflectorless chamber7, e.g. the (relatively small) diameter of the reflectorless chamber 7compared to the wavelength of the acoustic field. This particulardistribution enables the droplet 10 to be detached, accelerated andejected out of the reflectorless levitator/acoustophoretic printingapparatus and to be printed on any object/(printing) substrate 11.

Further, a drive means 5, e.g. a syringe pump as shown in FIG. 1, isused to feed the nozzle 6, via a tubing 4, with droplets on-demand sothat a highly-precisely controlled droplet ejection is possible. Thetubing 4 may be flexible, and, for instance, the tubing 4 may be madefrom flexible plastics.

Proof of principle experiments with water and glycerol confirmed thedroplet ejection (FIG. 3). For the experiments, a reflectorless chamber7 diameter of 2 mm and with a length of 5 mm was used. The emitterdiameter was set to 17 mm (circular), the height of the reflectoremitter (shaped as shown in FIG. 1) was 7 mm, the nozzle tip 6 a wasplaced at Hh=0.7 (i.e. in the lower part of the reflectorless chamberdiameter 7). The distance H between the upper surface 2 a of thereflector 2 and the lower surface 1 a of the emitter 1 was 0.52λ. Adrive means 5, a syringe pump, imposed a flow rate through the nozzle 6,while the acoustic forces 3 a, 8 a can control the droplet sizedetachment up to 3 orders of magnitude with a single 50 μm diameternozzle (75 μm<Rs<650 μm). The ultrasonic frequency, which was used inthis example experiment, was set to 25 kHz. With the interplay of flowrate and acoustic force 3 a, 8 a, ejection frequency can go as high as 2kHz in the DOD mode or several kHz in continuous jetting mode.Additionally, the extrusion mode can also be potentially used, since theink flows through a nozzle 6.

Further, in addition to the experimental results, a numerical simulationwas carried out. FIG. 10 presents results of an exemplary numericalsimulation of the acoustic pressure (in Pascal) within a preferredconfiguration of an acoustophoretic printing apparatus. In thisparticular example case, the maximum acoustic pressure in the (primary)chamber (between the emitter 1 and the reflector 2) is ≈3300 Pa, whilein the reflectorless chamber 7 the pressure is ≈15000 Pa. Since theacoustic forces 3 a, 8 a are proportional to the square of the acousticpressure, an enhancement of ≈125 times is expected.

Further, two preferred alternative configurations of the acoustophoreticprinting apparatus are depicted by FIG. 4.

Configuration A) of FIG. 4 shows that the nozzle 6 is introduced from aside (wall) of a chamber or open space between the emitter 1 and thereflector 2. The open space or chamber includes the first/main standingwave. The tubing 4 is bent so that the nozzle 6 can reach the conduit orreflectorless chamber 7, where a secondary acoustic standing wave or, atleast, a second acoustic field 8 is present.

Configuration B) of FIG. 4 shows that the nozzle 6 or the tubing 4enters from a hole 14 on a top of the emitter 1 and ends in thereflectorless chamber 7.

Furthermore, it is noted that different combinations are possible:emitter 1 and reflector 2 (as seen in FIG. 1), or emitter 1 and emitter1 (see e.g. FIG. 6). The emitter 1 and reflector 2 can be placed eitherup or down as indicated in FIG. 4 by the use of the combined referencesigns “1/2” at the positions of the emitter 1 and the reflector 2.

The fluids (first and second fluid) can be any fluid at any temperaturethat preferably does not spontaneously change their thermodynamic state(liquid and gas). Typically, the acoustophoretic printing system wouldwork in air and would eject liquids. The air can be at high or lowtemperature. In principle, the same system can be designed to performcontrolled ejection of liquids in immiscible liquids (the acousticmedium can be oil, for instance). The acoustic impedance of material maybe carefully determined.

The geometry of reflector 2 and emitter 1 can be axi-symmetric,two-dimensional (extrusion in the plane) or other geometries that allowfor the formation of the standing wave. Typically, an emitter 1 withcircular, squared, hexagonal or rectangular (cross-)section would beused and a reflector 2 with a characteristic dimension of its(cross-)section of the same order of magnitude.

The distance H (FIG. 4) between emitter 1 and reflector 2, morespecifically between the relevant surfaces of the emitter 1 and thereflector 2 can be a multiple of λ/2±0.2λ, such as 0.5λ±0.2λ, 1.0λ±0.2λ,1.5λ±0.2λ, etc. In such a way, an acoustic standing wave is imposedbetween the emitter 1 and reflector 2. Typically, a distance H≈λ/2offers the strongest acoustic standing wave. The relevant surfaces ofthe emitter 1 and the reflector 2 in the configuration of FIG. 1 are thelower surface 1 a of the emitter 1 and the upper surface 2 a of thereflector 2.

The emitter(s) 1 oscillate(s) periodically with any wave form (typicallya sinusoidal shape in ultrasonic range). The oscillation frequency canbe in the range from 10 Hz to 100 MHz. The wavelength λ is in the range1 μm to several meters. Typically, frequencies above 16 kHz (close tothe ultrasound range=not audible by human earing) and below 1 MHz offerthe best compromise between acoustophoretic printing feature size,strength of acoustic forces, component manufacturing. The oscillationvelocity amplitude can range from 1 μm/s to 100 m/s. The emitters 1 caninclude or be a piezoelectric transducer, a magnetostrictive transduceror other systems that can provide the needed wave excitation.

A characteristic dimension T of the reflectorless chamber/conduit 7(i.e. the diameter if it is a circular geometry, or the side length ifit is square geometry, etc.) can be in the range of 0.01λ to λ. Thereflectorless chamber 7 typically has a constant characteristicdimension T along an ejection line/printing axis A: preferably, thereflectorless chamber 7 is a cylindrical section with a constant radius(as shown in FIG. 4). The reflectorless chamber 7 may also be conical,increasing the T-value while exiting the reflectorless chamber 7. Ingeneral, a larger T at the exit or bottom 9 of the reflectorless chamber7 allows for a more stable droplet ejection. A height Hh (FIG. 4) of thereflectorless chamber 7 can be in the range from 0.01 λ to 100 λ.Typically, a preferred height Hh range for reliable and stable ejectionmay be up to 10λ.

A nozzle tip 6 a or its opening 6 b can have a diameter ranging from0.01 μm to several centimeters. The tubing 4 connected to the nozzle 6can have a diameter ranging from 0.1 μm to several centimeters.Typically, nozzle tips/openings 6 a, 6 b with diameters in the range of1 μm to 250 μm offer the best compromise between the minimumacoustophoretic printed feature size and pressure drops within thenozzle 6.

The nozzle tip 6 a can be positioned in any place within thereflectorless chamber 7, in particular, where a net force of theacoustic force 8 a is present. However, if Hh is the height of thereflectorless chamber 7, one of the preferred regions to place thenozzle tip 6 a for a reliable ejection is between the exit 9 of thereflectorless chamber 7 and Hh/3. The nozzle tip 6 a can be of anymaterial. Typically, it can be a tapered glass capillary, a Tefloncapillary or a microfabricated tube. If a glass capillary is used, it isusually useful to carry out a hydrophobic treatment, depending on theink that is printed. For water-based inks (e.g. most of the biologicalsolutions), the wetting of the nozzle tips by the inks is reduced byusing hydrophobic treatment. This also advantageously reduces thecapillary force Fc.

To advance the ink through the nozzle 6, a drive means 5, such as a backpressure system would suffice. Typically, it can be a syringe-pump orpressure controlled (second fluid) reservoir.

Steps of the printing method for ejecting the second fluid from thenozzle (6) include the forming of the droplet 10 at the nozzle tip 6 a.Preferably, the droplet 10 is formed by applying a pressure on thesecond fluid. An acoustic field 3, 8 with a force gradient, which pointsfrom the nozzle tip 6 a to the printing substrate 11, isgenerated/activated by an oscillating emitter 1. The actual detaching ofthe droplet 10 may be actuated by activating the emitter oscillations orby modulating the oscillations to build up sufficient acoustic forces 3a, 8 a. After the detachment, it is possible to deactivate theoscillations again or to modulate the oscillations such that theacoustic force 3 a, 8 a cannot detach a droplet 10 from the nozzle 6.Further, the detaching may also be caused by the droplet reaching a sizeat which the net force of the acoustic field 3, 8 overcomes thecapillary forces, i.e. without modulating/activating the acoustic field3, 8.

FIG. 5 shows a further preferred configuration of an acoustophoreticprinting apparatus. Specifically, FIG. 5 shows a multiple nozzleconfiguration. By replicating a configuration, such as depicted in FIGS.1 and 4, multiple droplets 10 (also of different inks) may besimultaneously and/or serially ejected. The reflectorlesschambers/conduits 7 may be comprised in a single reflector-member orprint head, as shown in FIG. 5, and, alternatively, a plurality ofreflectors 2 may abut on each other so that the print head as shown inFIG. 5 is formed. The reflectorless chambers 7 and the printing axes Amay be vertically arranged, as shown in FIG. 4. Alternatively, as FIG. 5depicts, may be arranged with an angle of up to 90° with regard to thevertical axis. More specifically, FIG. 5 shows that the printing axes Aof the plurality of reflectorless chambers 7 are varied such that theprinted droplets 10 may be deposited on a single position on thesubstrate 11. The ejection angle allows for a fine printing resolutionat the substrate/target surface.

In other words, FIG. 5 shows that the acoustophoretic printing apparatusis amenable to parallelization through a multiple nozzle system. Thisallows increasing the output capability. This renders feasibleup-scaling and multiple-materials can be used in acoustophoretic3D-printing. The print head features a nozzle array with independentlycontrollable droplet size and the pitch between printed droplets.

FIG. 6 shows two emitters 1 (the use of more than two emitters ispossible) that are used to focus the acoustic fields 3 at a focuspoint/predetermined point in space. This allows generating a net forcewhich is large enough to detach and accelerate the pendant droplet 10.However, more preferably, the emitters 1 are arranged such as to form astanding wave without a reflector 2. To form a standing wave, in fact,at least two travelling waves travelling in a substantially oppositedirection are needed, and, e.g., they should to have a predeterminedphase difference to create a standing wave. Similarly, for instance, inthe emitter-reflector configuration the reflector acts as an emitterwith a specific phase. The reflected wave will have a phase depending onthe physical distance from the emitter 1.

FIG. 7 shows using even more (multiple) emitters 1 to enhance theacoustic radiation pressure/acoustic force in a single focalpoint/predetermined point in space; a standing wave is not necessarilycreated. The emitters 1 are driven by (electric drive) signals withdifferent phases and amplitudes. Alternatively, the emitters 1 can beused to produces one/multiple vortex beam(s) acting on the pendantdroplet 10, generating a net force on it.

FIG. 8 shows a further configuration according to which an emitter 1 canbe designed so that it can act as a (integrated) reflector 2. Theemitter 1 has a cavity 15, into which the nozzle tip 6 a with thependant droplet 10 is inserted. The cavity is formed by the two sidewalls 16 of the emitter 1, which both oscillate from oppositedirections, creating a standing wave.

FIG. 9 shows, as one example, the use of the acoustophoretic printingapparatus as a sample dispenser for biological solutions in the nl-μlrange volume for standard 96, 384 and 1536 well plates 12.Configurations of the printing apparatus as shown in FIGS. 1, 4 and 5may be used. The droplets 10 are ejected into in the wells/grooves 13 ofthe well plate 12, e.g., for further inspection/treatment.

Furthermore, the herein-described the acoustophoretic printingmethod/apparatus may be used in any field of printing. In particular,the advantage of the Z-number freedom offered by the acoustophoreticprinting drastically broadens the material choice. Example fields ofapplication are:

1) Application of the acoustophoretic printing to high-Z inks (from 1 to10⁴). A sample dispenser of biological solutions in the nL-μL rangevolume for standard 96, 384 and 1536 well plates 12 (FIG. 6).

2) Application of the acoustophoretic printing to low-Z inks (from 10⁻⁴to 1). The capability of ejecting DOD highly viscous fluid allows theuse of conducting colloids with the purity of the inkjet inks but withthe conductivity and metal concentration of contact printing technology.The application can be extended to the nontoxic, conductive, low meltingpoint alloys as the eutectic gallium-indium, material of growinginterest for soft electromechanical system.

3) 3D-printing using Newtonian fluids: 3D-structures by DODacoustophoretic printing of fused deposition modeling (FDM) materials.This system would require a heated nozzle for melting commerciallyavailable FDM thermoplastics in filament form.

4) 3D-printing using non-Newtonian fluids: 3D-structures by DODacoustophoretic printing of thixotropic fluids. Ceramic, polymeric andmetallic inks, now printed only in filament forms due to theirshear-thinning behavior, would be DOD dispensed, paving the way for newfunctional printing.

5) 3D-printing of tissue engineering: hydrogel bioinks are the keymaterials for engineering complex human tissues. Bioinks engineered forextrusion printing can be printed with the acoustophoretic DOD printingtechnology.

6) Microparticle production: compared to microfluidics, in which animmiscible oil phase is necessary for droplet (particle) creation, theacoustophoretic ejection employs surface forces in air. The use of a gasas external medium (first fluid) allows the production of complexmicroparticles. Additionally, by using a nozzle characterized by acoaxial flow, Janus and hollow microparticles can be acoustophoreticallyproduced.

7) Biochemical analytical measurements: precise and rapid metering ofchemicals and assays for rapid chemical, biochemical and biologicalreactors.

Summarizing, the described acoustophoretic printing allows a controlledejection of predetermined volumes by use of acoustophoretic forces, atcontrolled frequency of droplet ejection. This can be used in diverseapplications such as a biological dispenser, as a two dimensional orthree-dimensional (3D) printer and for the production of microparticles.The printing process is not dependent from the material properties ofthe second fluid.

1. A printing apparatus comprising an emitter arranged within a first fluid and configured to oscillate for generating an acoustic field in said first fluid; a nozzle with a nozzle tip placed at a predetermined position within said acoustic field; a second fluid within the nozzle, and a driving means configured to drive a predetermined volume of the second fluid out of the nozzle.
 2. The printing apparatus according to claim 1, characterized in that a reflector is arranged between the emitter and a printing substrate.
 3. The printing apparatus according to claim 2, characterized in that a surface of the reflector is positioned at a distance (H) to the emitter, wherein the distance (H) is a multiple of λ/2±0.2λ, with λ being the wavelength of sound waves of a first acoustic field between the emitter and the reflector surface.
 4. The printing apparatus according to claim 2, characterized in that the reflector has a reflectorless chamber which is arranged along a printing axis (A) between the nozzle tip and the printing substrate, wherein the reflectorless chamber is a through-hole in a body of the reflector.
 5. The printing apparatus according to claim 1, characterized in that the printing apparatus comprises a plurality of emitters, wherein the emitters are arranged such that emitted sound waves are focused on a predetermined point in space within the acoustic field, and the predetermined position of the nozzle tip is at said predetermined point in space or in the vicinity thereof.
 6. The printing apparatus according to claim 1, characterized in that the acoustic field has a force gradient of acoustic forces, and the predetermined position of the nozzle tip is at a point or in the vicinity of a net force that pulls at the droplet pendant at the nozzle tip.
 7. The printing apparatus according to claim 4, characterized in that the predetermined position of the nozzle tip is within the reflectorless chamber in a region from an exit of the reflectorless chamber to Hh/3, with Hh being a height of the reflectorless chamber.
 8. The printing apparatus according to claim 1, characterized in that a plurality of nozzles and/or reflectorless chambers are arrayed for forming a multi-nozzle print head.
 9. The printing apparatus according to claim 1, characterized in that the nozzle has a heating means and/or a cooling means for heating/cooling the second fluid to a predetermined temperature.
 10. The printing apparatus according to claim 1, characterized in that a print head, which can be the emitter or the reflector, comprises a plurality of reflectorless chambers.
 11. The printing apparatus according to claim 1, characterized in that the nozzle and the drive means are connected via a tubing, wherein the tubing is introduced into the printing apparatus at a side surface thereof below the emitter or through a hole in the emitter.
 12. The printing apparatus according to claim 1, characterized in that the predetermined volume ranges from nl to μl.
 13. The printing apparatus according to claim 1, characterized in that the emitter is configured to oscillate with a frequency of 1 Hz to 1 GHz.
 14. The printing apparatus according to claim 1, characterized by further comprising a control means configured to control at least the oscillation of the emitter, the pressure application of the drive means and/or a driving of the printing apparatus to a printing position in relation to a printing substrate.
 15. A method for ejecting a second fluid from a nozzle which is arranged within a first fluid, comprising the step of forming a droplet at a nozzle tip by driving the second fluid out of the nozzle tip that is arranged within an acoustic field generated by an oscillating emitter. 